1. Field of the Invention
The present invention relates to ceramic electronic components and methods for making the same. In particular, the present invention relates to a ceramic electronic component having superior impedance characteristics in high-frequency bands such as the gigahertz band and exhibiting high impedance over a broad frequency range, and relates to a method for making the same.
2. Description of the Related Art
With use of electronic devices in high-frequency ranges in recent years, inductors, LC composite electronic components, LR composite electronic components, and LCR composite electronic components that can be used in the gigahertz band are required.
Unfortunately, stray capacitance generated parallel to a coil in inductors for high-frequency bands significantly affects the impedance of the coil. In particular, a minute stray capacitance in the range of 1/100 pF to 1/10 pF significantly affects the impedance in the gigahertz band. If the stray capacitance is reduced while required characteristics are maintained, the dielectric constant ∈ of the ferrite used as a magnetic material should be reduced. However, it is virtually impossible for the dielectric constant ∈ of the ferrite to be reduced to, for example, 13 to 14, because of the structural factor of the ferrite.
A possible way to reduce the dielectric constant ∈ is a composite magnetic material that is prepared by compounding of a material having a low dielectric constant, such as resin or glass, with a magnetic material. In such composite magnetic material, the magnetic particles are covered by the resin or glass nonmagnetic material. Since the magnetic path is segmented, the permeability of the composite magnetic material is extremely low.
A porous ferrite sintered compact having a porosity of 20% to 70% is known as a ferrite material having a low dielectric constant which is used in an electromagnetic wave absorber, as disclosed in Japanese Unexamined Patent Application Publication No. 55-526300. Since this ferrite sintered compact containing a high rate of pores has a low dielectric constant and a continuous magnetic path, it does not cause a discontinuous and large variation in electromagnetic characteristics. In the porous ferrite sintered compact, individual ferrite particles are magnetically coupled with each other even at a high porosity; hence, this ferrite sintered compact exhibits a small dependence of complex permeability on frequency dispersion characteristics. However, a high porosity of this porous ferrite sintered compact causes decreased mechanical strength, and more particularly, low flexural strength, of a resulting electronic component.
A porous ceramic electronic component is disclosed in Japanese Unexamined Patent Application Publication No. 11-67575. This ceramic electronic component is composed of ceramic containing 3% to 30% by volume of pores with a diameter of 1 xcexcm to 3 xcexcm and an internal electrode provided in the ceramic. Such a porous ceramic electronic component has a low dielectric constant and thus exhibits improved impedance characteristics.
In this technology, the upper limit of the pore content is 30% by volume because a pore content exceeding the upper limit decreases the flexural strength of the ceramic. Thus, the relative dielectric constant cannot be reduced to a level that is required for current ceramic electronic components having superior characteristics. Furthermore, the pores in the ceramic of this ceramic electronic component easily absorb moisture. A high water absorbance deteriorates reliability of the component.
An object of the present invention is to provide a ceramic electronic component including a ceramic sintered compact having a low dielectric constant and high mechanical strength and a method for making the ceramic electronic component.
According to a first aspect of the present invention, a ceramic electronic component comprises a ceramic sintered compact containing about 35 to 80 volume percent pores; and an electrode provided inside the ceramic sintered compact, wherein the pores are fully or partly filled with resin or glass.
The ceramic electronic component contains about 35 to 80 volume percent pores that are fully or partly filled with the resin or glass. Furthermore, the ceramic phase of the ceramic sintered compact is continuous in the present invention. Accordingly, the ceramic electronic component exhibits a decreased dielectric constant without deterioration of electrical characteristics and mechanical strength of the ceramic sintered compact.
The present invention is based on the recognition by the inventors that a small amount of resin or glass packed in the pores significantly improves the tensile stress of ceramic materials have high compression stress but low tensile stress, such as ferrite. Thus, a ceramic electronic component having a porosity of about 80 volume percent according to the present invention favorably compares with a conventional component having a porosity of 30 volume percent in that the dielectric constant is drastically decreased to about 6 or less without deterioration of the mechanical strength such as flexural strength and electrical characteristics.
The ceramic electronic component of the present invention has pores in the ceramic sintered compact. If the ceramic sintered compact is magnetic, the permeability of the porous magnetic sintered compact decreases to some extent compared with the corresponding solid compact. However, the magnetic ceramic sintered compact has a continuous magnetic path. Thus, the permeability of the magnetic material is maintained in the sintered compact. This indicates that the cross-point frequency at which xcexcxe2x80x2=xcexcxe2x80x3 does not substantially change.
Preferably, the average pore diameter is in the range of about 5 xcexcm to 20 xcexcm. A diameter of less than about 5 xcexcm causes the formation of closed pores that cannot be impregnated with glass or resin. A diameter exceeding about 20 xcexcm causes a noticeable reduction in strength of the sintered compact. More preferably, the average pore diameter is in the range of about 5 xcexcm to 10 xcexcm.
The porosity (volume rate) of the pores must be at least about 35 volume percent in order that the ceramic sintered compact has a sufficiently low dielectric constant with satisfactory mechanical strength. However, a porosity exceeding about 80 volume percent causes a noticeable decrease in mechanical strength that inhibits subsequent processing such as resin or glass impregnation. Thus, the upper limit of the porosity is about 80 volume percent in the present invention. Preferably, the porosity is in the range of about 40 volume percent to about 50 volume percent. Since the resin or glass packed in the pores reinforces the ceramic sintered compact, the ceramic sintered compact can be processed within the pore diameter range of about 5 xcexcm to 20 xcexcm and the porosity of about 80 volume percent or less.
Preferably, the ceramic sintered compact is formed by firing a green compact comprising a ceramic raw material, a binder, and a spherical or granular combustible material having adhesiveness to the binder. If a magnetic sintered compact is formed in this process, the porous ceramic sintered compact has a continuous magnetic path. Thus, the ceramic electronic component has desired magnetic characteristics, reduced stray capacitance, and required electrical and mechanical characteristics.
Preferably, the resin or glass in the pores contains internal pores. The internal pores contribute to a further decrease in dielectric constant of the ceramic sintered compact.
Preferably, the ceramic sintered compact is a magnetic ceramic sintered compact. The magnetic ceramic material can be used for production of inductors. For example, an inductor as a ceramic electronic component comprising the magnetic ceramic sintered compact has desired magnetic characteristics, reduced stray capacitance due to a decreased dielectric constant, and required electrical and mechanical characteristics.
In the present invention, the ceramic electronic component may an inductor, an LC composite electronic component comprising an inductor segment and a capacitor segment, an LR composite electronic component comprising an inductor segment and a resistor segment, and an LCR composite electronic component comprising an inductor segment, a capacitor segment, and a resistor segment, and the like. These ceramic electronic components exhibit high mechanical strength, reduced stray capacitance, and other desired properties.
The ceramic electronic component of the present invention may have a multilayer structure including an electrode layer provided between ceramic layers. That is, the present invention is preferably applicable to a multilayer or monolithic ceramic electronic component since the ceramic sintered compact of the present invention has high reliability, i.e., high mechanical strength such as flexural strength, and a low dielectric constant.
Preferably, at least one surface of the ceramic sintered compact is also covered with the resin or glass. More preferably, all the surfaces are covered with the resin or glass. The surface resin or glass layer or layers reinforce the ceramic sintered compact.
According to another aspect of the present invention, a method for making a ceramic electronic component including a ceramic sintered compact and an electrode inside the ceramic sintered compact, comprises the steps of:
(a) forming a green compact including an electrode therein with a ceramic compound comprising a ceramic raw material, a binder, and a spherical or granular combustible material having adhesiveness to the binder;
(b) firing the green compact to form the ceramic sintered compact including the electrode and containing about 35 to 80 volume percent pores; and
(c) impregnating the pores of the ceramic sintered compact with resin or glass.
The ceramic electronic component produced by this method contains about 35 to 80 volume percent pores that are filled with the resin or glass. Furthermore, the ceramic phase of the ceramic sintered compact is continuous in the present invention. Accordingly, the ceramic electronic component exhibits a decreased dielectric constant without deterioration of electrical characteristics and mechanical strength of the ceramic sintered compact. The resin or glass packed in the pores reinforces the ceramic sintered compact. Accordingly, this method allows production of ceramic electronic components having satisfactory mechanical, electrical and magnetic characteristics.
The combustible material may be spherical or granular including powder. Preferably, the combustible material is spherical from the viewpoint of dispersion uniformity. Preferably, the combustible material has an average particle size in the range of about 5 xcexcm to 20 xcexcm in order to form pores with a diameter in the range of about 5 xcexcm to 20 xcexcm. The combustible material is added to the ceramic compound in an amount of about 35 volume percent to 80 volume percent and preferably about 40 volume percent to 50 volume percent to form a desired porosity. Within the range, the combustible material content may be appropriately determined depending on a target porosity.
Preferably, the combustible material comprises at least one compound selected from the group consisting of crosslinked polystyrene, crosslinked polymethyl methacrylate, crosslinked polybutyl methacrylate, crosslinked polymethacrylate esters and crosslinked polyacrylate esters. These combustible materials can readily burn in the firing step and thus facilitate the formation of a ceramic sintered compact with a desired porosity. For a significantly high porosity, the combustible material content should be generally increased whereas the binder content should be decreased. This, however, causes a decrease in mechanical strength of green products, resulting in a low product yield. Crosslinked polymer with a large surface area is preferred because it exhibits high adhesiveness to the resin binder and high shape retention. Thus, the binder content can be reduced in the present invention. Accordingly, the method facilitates production of a ceramic electronic component comprising a ceramic sintered compact with a high porosity at a high product yield.
In the method, the resin or glass may contain a solvent or a diluent. In this case, in the step (c), the pores are filled with the resin or glass, and then the solvent or diluent is evaporated to form internal pores in the resin or glass. The internal pores contribute to a further reduction in dielectric constant of the ceramic sintered compact. Examples of usable solvents or diluents are ethanol, xylene, butyl acetate and water.
Preferably, the solvent (diluent) content is in the range of about 5 to 50 parts by volume to 100 parts by volume of resin or glass. A solvent content of less than about 5 parts by volume causes an insufficient decrease in viscosity of the solution and thus precludes processing. A solvent content exceeding about 50 parts by volume results in insufficient removal of the solvent and residual solvent in the resin deteriorates properties of the resin.
In the method, the resin or glass may be partly soluble in a solvent. In this case, the pores are filled with the resin or glass in the step (c), and then the resin or glass is partly dissolved with the solvent to form internal pores in the resin or glass. Since the resin or glass packed in the pores of the ceramic sintered compact is partly removed in this step, the ceramic sintered compact has a smaller dielectric constant. In this case, the resin or glass itself need not be soluble in the solvent. For example, a component compounded in the resin may be soluble in the solvent. Alternatively, the glass may contain a soluble component so that the glass can be partly removed when the soluble component is dissolved into the solvent.
Preferably, the ceramic raw material is a magnetic ceramic raw material. For example, a magnetic ceramic material is used for production of inductors. An inductor as a ceramic electronic component comprising the magnetic ceramic sintered compact has desired magnetic characteristics, reduced stray capacitance due to a decreased dielectric constant, and required electrical and mechanical characteristics.
In the present invention, the ceramic electronic component may be an inductor, an LC composite electronic component comprising an inductor segment and a capacitor segment, an LR composite electronic component comprising an inductor segment and a resistor segment, or an LCR composite electronic component comprising an inductor segment, a capacitor segment, and a resistor segment. These ceramic electronic components exhibit high mechanical strength, reduced stray capacitance, and other desired properties.
In the step (a), the green compact may formed by providing an electrode layer on one or more of a plurality of green ceramic sheets comprising the ceramic compound, and stacking the plurality of green ceramic sheets having the electrode layer. According to this process, a monolithic ceramic electronic component having high reliability and high mechanical strength can be readily produced with high efficiency.